The force of tension is a pulling force exerted by a rope, string, or cable on an object it is attached to. It acts along the direction of the rope and can be used to transmit forces between objects or support loads.
Normal force refers to the support force exerted by a surface that is perpendicular (normal) to that surface. For example, when standing on the ground, your weight is balanced by the normal force pushing up from the ground.
Frictional Force: Frictional force refers to resistance encountered when two objects move against each other. In this context, frictionless means there is no frictional force acting between surfaces.
Equilibrium occurs when all forces acting on an object balance out and there is no net acceleration. In this case, if block A isn't accelerating vertically, then the magnitude of gravitational force pulling it down must be balanced by an equal magnitude of tension pulling it up.